This invention relates to a unique sensing device which is known commercially and described in the literature under the trademark "Metritape" sensor, and which is the subject of several U.S. patents, including U.S. Pat. Nos. 3,511,090; 3,583,221; and 3,792,407.
The Metritape sensor comprises an elongated metallic base strip having electrical insulation on the edges and back of the strip to define an uninsulated channel along the length of the base strip, and a resistance wire or ribbon helically wound around the insulated base strip, with the helical turns bridging the insulated edge portions being spaced from the underlying uninsulated channel of the base strip. This sensor structure is enclosed within a continuous polymeric or other protective sleeve to provide a clean and dry inner chamber for the sensor. The sensor is disposed within a tank or vessel containing the liquid or fluent material, the level of which is to be monitored. The pressure of the material surrounding the immersed sensor causes the deflection of the helical turns in the immersed portion of the sensor into engagement and electrical contact with the underlying base strip, such that an electrical resistance proportional to material level is provided.
Applications for this elongated resistive sensor have ranged from the gauging of deep oil and ballast tanks on ocean-going supertankers, to land-based tanks in which turbulent and agitated slurries are held, to more quiescent tankage in which petroleum and chemical products are stored. Within such application environments, the elongated sensor strip may be subjected to shock, vibration, sudden impacts, and scraping and tearing forces which result from contact of the sensor with surrounding structure. Furthermore, the pipe, channel, tank or sump in which the sensor is housed may have sharp edges, corners, threads, welds improperly made or inadequately ground, or other structural protrusions against which the sensor may strike or chafe.
It is, furthermore, desirable that the outer sheath of the elongated resistive sensor be impervious and resistant to chemicals, solvents, slurries and suspensions in which it is immersed. It is also required that the sensor outer envelope be compliant and responsive to external material pressures so that the air/liquid interface between the material and the void space above the material can be precisely and repeatedly located by the sensor.
To achieve this combination of requirements, which include sensitivity, ruggedness and resistance to corrosion, a polymeric material such as Teflon fluorocarbon, or an olefin such as polyethylene or polypropylene, may be employed as the outer jacket envelope. Such polymers, while having excellent chemical properties, are susceptible to wear, particularly when they are brought into abrasive contact with an angular steel structure. Under severe conditions of use, it may be necessary, assuming inadequate protection is provided, to replace the sensing device at periodic intervals, thus raising the cost for such installation in terms of both replacement parts and the labor required to perform the replacement.
Conditions of usage can be particularly severe when these sensing devices are used on board ocean-going vessels which may encounter severe wave and mechanical-motion conditions in open sea. Because such conditions may occur when the tanks are empty of product, there will then be no material present to provide viscous damping and cushioning of the sensor from the interior of a close-fitting steel pipe in which it may be contained.
Several techniques have been employed to provide protection for elongated resistive sensors during their years of varying usage. The sensors have been mounted in close-fitting, elastomeric tubes, or hoses, which have soft, non-metallic interior walls, this being a structure which may be employed when elongated resistive sensors are suspended down deep observation wells.
A more common protective means has been C-shaped channel, extruded in a configuration that captures the edges and the back of the sensor, but leaves the front face open for direct access and compression by surrounding fluent material. The resulting channel or bumper strip, provides protection to the sensor edges and back, but is difficult to clean and is expensive to extrude in the most corrosion resistant of polymer materials, such as Teflon fluorocarbon polymer.
The subject invention represents a means for achieving sensor edge protection by using highly qualified materials in forms that are readily available from commercial sources, making it unnecessary to develop specialized extruding and shaping dies, and other such costly tooling, made exclusively for this particular application.
Accordingly, it is an object of this invention to create a material level-sensing device which is highly responsive to the actuation pressures it must receive, and yet is protected against adverse damaging contact and abrasion.
Another object is to achieve such properties, using protective means that are readily available in forms that require only simple modification for their use.
A further object is to employ means that are available in a wide range of advantageous materials, in a selection of sizes and configurations, and in long unspliced lengths, resulting in application suitability and in minimum material waste.
Another object is to achieve a means for protecting the edges of a precision elongated sensor structure, having such shape as to cause the sensitive faces of the structure to stand away from potentially damaging surface and edges and thereby to reduce the potential for wear and damage.
Yet another object is to have protective means available in standardized form and in such dimensions that, when added to the underlying product, the overall dimensions are within the size of available access openings, allowing the enhanced structure to be used in both existing and future installations.
An additional object is to acquire edge protection materials having both shape and Physical properties that make them easy to modify for their application and which, when applied to the underlying product, do not distort it, damage it or otherwise interfere with its full and proper functioning.
Yet another object is to provide edge-protective means in such form and flexibility that it can be progressively attached to the underlying structure by simple hand tool, both in the factory and in the field, and at a pace that makes this enhancement economically attractive.
Another objective herein is to apply edge protection to an elongated material-level transducer such that the combined and assembled structure can be easily coiled for storage, packing and shipment, and such that coiling causes minimal relative motion and wear between the protective elements and the underlying sensor itself.
Another objective of this invention is to provide a protective means which has minimal interior volume for entrapment and retention of materials, for use in food and other applications where the retention and carryover of gauged material must be minimal.
Yet another object of this invention is to achieve a friction engagement force between the add-on reinforcement and the outer envelope of the transducer itself that is sufficient to retain the protective elements in place throughout usage, but which still allow easy removal, and even reuse, of the protective elements when such is desirable.